1MPXIO(5)                      File Formats Manual                     MPXIO(5)


6       mpxio - multiplexed i/o


9       #include <sys/mx.h>
11       #include <sgtty.h>


14       Data  transfers on mpx files (see mpx(2)) are multiplexed by imposing a
15       record structure  on the  io  stream.   Each  record  represents   data
16       from/to  a particular channel or a control or status message associated
17       with a particular channel.
19       The prototypical data record read from an mpx file is as follows
21            struct input_record {
22                 short     index;
23                 short     count;
24                 short     ccount;
25                 char data[];
26            };
28       where index identifies the channel, and count specifies the  number  of
29       characters  in  data.  If count is zero, ccount gives the size of data,
30       and the record is  a control or  status  message.   Although  count  or
31       ccount might be odd, the operating system aligns records on short (i.e.
32       16-bit) boundaries by skipping bytes when necessary.
34       Data written to an mpx file must be formatted as  an  array  of  record
35       structures defined as follows
37            struct output_record {
38                 short     index;
39                 short     count;
40                 short     ccount;
41                 char *data;
42            };
44       where  the data portion of the record is referred to indirectly and the
45       other cells have the same interpretation as in input_record.
47       The control messages listed below may be read from a  multiplexed  file
48       descriptor.   They are presented as two 16-bit integers: the first num‐
49       ber is the message code (defined  in  <sys/mx.h>),  the  second  is  an
50       optional parameter meaningful only with M_WATCH and M_BLK.
52            M_WATCH - a process `wants to attach' on this channel.  The second
53                 parameter is the 16-bit user-id of the process that  executed
54                 the open.
55            M_CLOSE  -  the channel is closed.  This message is generated when
56                 the last file descriptor referencing  a  channel  is  closed.
57                 The  detach command (see mpx(2) should be used in response to
58                 this message.
59            M_EOT - indicates logical end of file on a channel.  If the  chan‐
60                 nel is joined to a typewriter, EOT (control-d) will cause the
61                 M_EOT message under the conditions specified  in  tty(4)  for
62                 end  of file.  If the channel is attached to a process, M_EOT
63                 will be generated whenever the process writes zero  bytes  on
64                 the channel.
65            M_BLK  -  if  non-blocking  mode  has  been enabled on an mpx file
66                 descriptor xd by executing ioctl(xd, MXNBLK, 0), write opera‐
67                 tions  on the  file are truncated in the kernel when internal
68                 queues become full.  This is done on a per-channel basis: the
69                 parameter  is  a count of the number of characters not trans‐
70                 ferred to the channel on which M_BLK is received.
71            M_UBLK - is generated for a channel after M_BLK when the  internal
72                 queues have drained below a threshold.
74       Two  other messages may be generated by the kernel.  As with other mes‐
75       sages, the first 16-bit quantity is the message code.
77            M_OPEN - is generated in conjunction  with  `listener'  mode  (see
78                 mpx(2)).   The uid of the calling process follows the message
79                 code as with M_WATCH.  This is followed by a  null-terminated
80                 string which is the name of the file being opened.
81            M_IOCTL  -  is generated for a channel connected to a process when
82                 that process executes the ioctl(fd, cmd, &vec)  call  on  the
83                 channel file descriptor.  The M_IOCTL code is followed by the
84                 cmd argument given to ioctl followed by the contents  of  the
85                 structure  vec.   It is assumed, not needing a better compro‐
86                 mise at this time, that the length of vec  is  determined  by
87                 sizeof (struct sgttyb) as declared in <sgtty.h>.
89       Two control messages are understood by the operating system.  M_EOT may
90       be sent through an mpx file to a channel.  It is equivalent  to  propa‐
91       gating  a  zero-length  record through the channel; i.e. the channel is
92       allowed to drain and the process or device at the other end receives  a
93       zero-length  transfer  before  data  starts flowing through the channel
94       again.  M_IOCTL can also be sent through  a  channel.   The  format  is
95       identical to that described above.
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